1. Field of the Invention
This invention lies in the field of underground pipeline cathodic protection.
More particularly, it is concerned with apparatus for detecting, by measurement above the surface of the earth, the flow of electrical current in a pipeline below the surface of the earth.
It is still further concerned with the measurement of current in a pipeline below the surface of the ground, by determination of the difference in flux in two identical magnetic flux detectors, which are symmetrically placed above the earth, one on each side of the buried pipeline.
2. Description of the Prior Art
In any industry, such as the oil industry, which utilizes underground pipelines, it is well known that a ferrous pipe will corrode under certain circumstances of moisture and impurities in the metal of the pipe, etc. To counteract this electrolytic corrosion, pipelines are often coated with an electrically insulating material which is securely bonded to the pipe so as to avoid the passage of an ionic current from the pipe to the earth surrounding it.
It is also well known that, in other circumstances, it is the custom to provide a source of DC voltage between the earth and the pipe where the pipe is negative with respect to the grounded end of the source. Thus a current is set up to flow from the earth to the pipe, this current being larger than any possible current in the reverse direction, which might be set up due to electrolytic conditions. Consequently, since there is no flow of ionic current from the pipe to the earth, there will be no ions travelling in that direction that would serve to remove metal atoms from the pipe, and therefore eventually cause corrosion and perforation of the pipe.
It is also customary under certain conditions to use cathodic protection, which comprises connecting a conductor between the pipe and a buried piece of metal of the type such as aluminum or magnesium, which has a lower electrochemical potential than the iron of the pipe, so that the electrolytic cell comprising the buried aluminum or magnesium, and the iron, with the moist earth as electrolyte, will cause a current to flow from the anode through the earth to the pipe, and so prevent the flow of ions in the opposite direction.
It is often found however that even with an electrically insulating coating on the pipe, that pinholes, breaks, or holidays in the coating are often accidentally or otherwise formed, which permit currents to flow from the pipe to the earth, and to set up in the pipe longitudinally flowing currents, which can be detected by the apparatus of this invention.